The conversion of solar energy into electrical energy utilizing photovoltaic devices historically has been considered to have only marginal utility. Early and current plate-type devices were somewhat small, non-concentrating and non-suntracking. Thus, their employment has been limited to remote applications, for example, recharging the batteries of control devices and the like at locations where conventional line power is not available. Considered on a cost per watt basis (about $6.00 per watt to about $8.00 per watt) their form of power generation is quite expensive.
In 1973, with the advent of the oil crisis, government funded efforts were undertaken to develop alternate energy sources including concentration-based photovoltaic systems and the harnessing of wind. While the technology for developing wind conversion systems was sufficient to make them ultimately practical, photovoltaic system technology lagged. However, several large-scale demonstration products were developed.
As the energy crisis passed and oil prices lowered, concentrator-based photovoltaic programs diminished. At the present time, while important improvements in concentrator-based photovoltaic systems have been developed, the cost of power generation produced by them remains non-competitive with fossil fuel-based generation. In 2000, a leading concentration-based photovoltaic investigator stated:                In reaching for the ultimate goal of providing clean, renewable energy, concentrators compete head-on with existing fossil fuel-fired generators. Projected electricity costs from concentrator power plants are about three times the current cost of energy from natural gas power plants. Early concentrator plants will be twice as expensive again. There is nothing that can be done about this without government involvement, period. We need to decide as a society if environmental issues such as acid rain, global warming, and reduced health are important enough to subsidize this difference for a while. Richard M. Swanson, “The Promise of Concentrators”, Prog. Photovolt. Res. Appl. 8, 93–111 (2000).        
One of the more interesting innovations in the photovoltaic arena has been the introduction of the high voltage silicon vertical multijunction (VMJ) solar cell. Sometimes referred to as an “edge-illumination” multijunction cell, the VMJ cell is an integrally bonded series-connected array of miniature silicon vertical junction cell units. This series connection aspect overcomes the low voltage characteristic of the cells. The devices were developed by a NASA scientist, Bernard L. Sater at the NASA Glen Research Center. They are described in U.S. Pat. No. 4,332,973 entitled “High Intensity Solar Cell”, issued Jun. 1, 1982 by Sater; U.S. Pat. No. 4,409,422 entitled “High Intensity Solar Cell”, issued Oct. 11, 1983 by Sater; and U.S. Pat. No. 4,516,314 entitled “Method of Making a High Intensity Solar Cell”, issued May 14, 1985 by Sater, which patents are incorporated herein by reference.
In consequence of the VMJ topology where several junctions are stacked on top of one another, a resulting voltage will be equal to the voltage of each junction times the number of junctions (N). Thus, energy for a given current is reduced by the factor, N, and the parasitic power losses are reduced by a factor of, N2. For example, where a VMJ is configured with forty junctions, the parasitic losses will be reduced by the factor 1/1600. This means that the cells can react to concentration solar intensities up to about 1600 times solar intensity of that of conventional cells operating at the same power level. It is this reduction in parasitic losses which permits the noted increase in the concentration levels at which such cells can operate and which thus further achieves a cost reduction in energy generation. As an example of the cells performance, a 0.78 cm2 VMJ cell with forty series connected junctions produced 31.8 watts at 25.5 volts at nearly 2500 suns AM1.5 intensities in flash tests at the NASA Glen Research Center. That demonstrates a VMJ cell output power density of 40.4 watts/cm2 with an estimated input of 211 watt/cm2 and an efficiency near 20%. The Arizona Public Service Company has used the cell in a 100 kW installation at the Glendale Arizona Municipal Airport. Further uses of the cells are underway in Australia.
Another innovation in concentration photovoltaic devices has been evolved in connection with the SunPower Corporation of Sunnyvale, Calif. In the 1980s, R. M. Swanson (supra) proposed a point contact solar cell capable of performing with concentrators. To accommodate the above-noted low voltage aspects of photovoltaic devices, multiple junctions of these small area cells are arranged in series in a monolithic semiconductor substrate. Such devices currently are referred to as “back surface point contact silicon solar cells”. The cells and their manufacture are described, for instance, in the following U.S. Patents which are incorporated herein by reference: U.S. Pat. No. 4,927,770 by Swanson, entitled “Method of Fabricating Back Surface Point Contact Solar Cells”, issued May 22, 1990; U.S. Pat. No. 5,164,019 by Sinton entitled “Monolithic Series-Connected Solar Cells Having Improved Cell Isolation and Method of Making Same”, issued Nov. 17, 1992; U.S. Pat. No. 6,274,402 by Verlinden et al., entitled “Method of Fabricating a Silicon Solar Cell”, issued Aug. 14, 2001; U.S. Pat. No. 6,313,395 by Crane et al., entitled “Interconnect Structure for Solar Cells and Method of Making Same”, issued Nov. 6, 2001; and U.S. Pat. No. 6,333,457 by Mulligan et al., entitled “Edge Passivated Silicon Solar/Photo Cell and method of Manufacture”, issued Dec. 25, 2001.
See additionally the following publications:                Verlinden et al., “Backside-Contact Silicon Solar Cells With Improved Efficiency for the '96 World Solar Challenge”, Proceedings of the 14th EC Photovoltaic Solar Energy, Barcelona, Jun. 30–Jul. 4th, 1997, pp 96–99.        Mulligan et al., “A Flat-Plate Concentrator: Micro-Concentrator Design Overview, Proceedings 28th EEE PVSC, 2000.        
Endeavors also have been witnessed which are concerned with multijunction cell design utilizing a combination of Perodic III-V semiconductor materials to capture an expanded range of photon energies. One concept in this regard has been to split the impinging spectrum to photovoltaicly engage semiconductor materials somewhat optimized to a split-off spectral band. An approach considered more viable has been to grow multiple layers of semiconductors with decreasing band gaps. Top layers of these devices are designed to absorb higher energy photons while transmitting lower energy photons to be absorbed by lower layers of the cell. For example, the National Renewable Energy Laboratory (NREL) design of a GalnP/GaAs/Ge triple-junction solar cell design has been reported achieving a conversion efficiency of 34% under concentrated light.
See generally, the online document by B. Burnett:                www.nrel.gov\ncpv\pdfs\11—20_dga basics—9–13.pdf        
In general, there are difficulties with the latter multi-spectral approach particularly associated with lattice-constant constraint and current matching. In the latter regard, the output current of the multijunction solar cells is limited to the smallest of the currents produced by any of the individual junctions. For this reason all junctions in the monolithic device must be designed to produce the same amount of photo current.
Among the drawbacks remaining associated with current concentration photovoltaic systems is the heat build-up occasioned by their relatively lower efficiencies. That heat is the result of ineffective photonic interaction with the cells, i.e., only a portion of the concentrated solar energy is converted at their depletion layers into useful energy, the rest being absorbed as heat throughout the cell. Further, the cells must be operated under restrictive temperature limits. While heat sinking is utilized to combat heat build-up, there are limits to heat sinking capabilities. Passively (convectional) cooled heat sinks have a heat sink limit of about forty watts/cm2, while active (water cooled) heat sinking has a heat sink limit of about eighty watts/cm2.